Silicone oils are widely used in the skin care and cosmetic industry. As noted, traditional silicone gel thickening systems (i.e., silicones which are themselves structured and can in turn be used to structure other benefit agents, such as petrolatum) typically use silicone based elastomers to structure the silicone oil. Formulation space in these systems is limited because, for example, addition of polar and non-polar benefit oils (e.g., to be delivered in compositions in which the structured silicone is used) may cause de-swelling of the silicone and/or phase separation.
Quite unpredictably, applicants have found that, if the silicone is instead structured with a unique gelling or structuring system (rather than traditional silicone gel thickener/gellants) comprising a semi-crystalline copolymer backbone having pendant side chains which form crystals to aid gelation, upon cooling (at temperature <35°, preferably <30° C.; melting point is above this, e.g., >35°-40° C.) and wherein >70%, preferably >75% of the pendant groups are alkane groups of certain minimum chain length grafted to a copolymer backbone, the silicone will provide the benefits of silicone (superior skin feel). At the same time these copolymers will more readily help deliver separate benefit actives (e.g., fatty acids such as stearic acid, petroselenic acid, oleic acid, linoleic acids; branched, saturated and unsaturated hydrocarbons such as squalane, squalene etc.) from a hydrocarbon phase which phase is formed by a combination of the copolymer grafted alkane chains (e.g., C20 group grafted to polyvinylpyrrolidone, or PVP) and the type of low-molar mass hydrocarbons defined as benefit actives above. That is, the pendant groups on the copolymer and the separate benefit agents together form the “hydrocarbon phase”. While not wishing to be found by theory, it is believed that thickening of the silicone is caused by formation of a gel network due to crystallization of the pendant alkyl chains (only the side chains crystallize while the backbone is amorphous).
The copolymer chain carrying the pendant alkyl groups may comprise, for example, (1) statistically random copolymer, graft-copolymers, or block copolymers; and (2) a cross-linked (network) copolymer which together form “backbone”. Other hydrocarbons (e.g., fatty acids, long chain alcohols and paraffins) can be added to the copolymer (forming part of hydrocarbon phase as noted above) as they are miscible with the side chains of the copolymer. As indicated, the benefit agent actives (e.g., fatty acids and other skin actives), if used, preferably interact with pendant groups on the polymer and/or cross-linked (network) copolymer (backbone) to form part of the hydrocarbon phase (i.e. form crystals, mesomorphic structures, and or phase-separated domains and microdomains), and help structure the silicone oil. In this way, the pendant alkyl groups perform multiple tasks. They help “thicken” silicone and better deliver the superior feel silicone; and they interact with optional benefit agent, to help better deliver these.
The “silicone gelling system” of the invention thus comprises a combination of the silicone oil; backbone copolymers bearing alkyl chains; and optional hydrocarbons (i.e., hydrocarbons which can separately provide benefit). While not wishing to be bound by theory, it is believed that using copolymers which have high content of long chain alkyl helps make the silicone gel sensitive to shear, and provides a unique gel fracture rheological profile (gel fracture is point where crystalline network is broken and gel becomes more liquid-like) having minimal viscosity after the gel is destroyed on the skin. This unique gel fracture mechanism is beneficial both for skin feel and to help active release on deposition off the structured silicone. The gel is also characterized by the fact that it retains shape when subject to earth gravity (i.e., flip over 100 g container to observe liquid), even after five minutes. In addition, these gels are phase stable after three months.
The gel system is also temperature sensitive, i.e., may soften on the skin and melt. It can behave similar to petrolatum in this sense, but with improved sensory (silicone sensory) and controlled release (based on gel fracture Theological profile noted by using this gelling system). It should be noted that the gelling system can comprise just the copolymers plus silicone oil. Separate hydrocarbons are not necessary to form gel, but are used to deliver skin care benefits.
In short, the mechanism of silicone gel formation and resulting gel so formed are new because the gel (formed by silicone oil and copolymer) is stabilized by semi-crystalline copolymer that carries at least 70% by weight alkane having chain length of C18-C30, preferably C20-C24. The ideal chain length for gelation within the range is really dependent on the composition of pendant chains (e.g., total amount) and molecular weight of overall polymer. One can also have effective gelation for C30, for example, provided that there are fewer pendant groups. However, since the melting temperature of alkyl chains increases very rapidly with their length, C20 is preferred from the processing stand point. In short, amount and chain length of hydrocarbon chains need to be selected so the polymer is soluble enough to form a gel but not so soluble as to phase separate. As indicated, copolymer may comprise optional separate hydrocarbon.
In general, silicone oils are used in many patents (e.g., U.S. Pat. No. 6,555,099 to Guskey et al.; U.S. Pat. No. 6,524,562 to Guskey). Copolymers like those used as gellant in the subject invention (e.g., U.S. Pat. No. 5,622,793 to Iijima et al.; U.S. Pat. No. 6,582,683 to Jezior) are also disclosed, but it is unpredictable they could be used as silicone structurants or what effect they would have.
As far as applicants are aware, there is no reference disclosing the copolymers comprising these long chain alkyl groups of the subject invention used as gellants or structurants of silicone oil (e.g., silicone polymer comprising such polymers); or the benefits associated with use of such unique gellant system. Further, such structured silicones would not form spontaneously and the process would have to be designed to ensure such silicones are formed. Therefore, the fact that these polymers yield such gels is quite unpredictable.